1. Field of the Invention
This invention relates to a semiconductor comprising a oxide film as an insulating film which has good insulating property and to a process for manufacturing such a oxide film.
2. Description of the Prior Art
Recently, there have been attempts to reduce an equivalent oxide thickness tox while maintaining a small leakage current for an insulating film, for achieving a further integrated, higher performance and further power-consumption saving semiconductor device. An equivalent oxide thickness tox means a value obtained by reducing a real thickness t of a film with a dielectric constant ε into a thickness of a silicon oxide film with a dielectric constant εSiO, as defined by the following equation.tox=εSiOt/ε
For example, a gate insulating film in a semiconductor transistor with a 0.05 μm generation of gate length requires an equivalent oxide thickness tox of 1 nm or shorter and a leakage current density of 10−6 A/cm2 or less at a gate voltage of 1.2 V.
A capacity insulating film in a semiconductor memory with a 0.13 μm generation of gate length requires an equivalent oxide thickness tox of 0.3 nm or shorter and a leakage current density of 10−8 A/cm2 or less at an applied voltage of 1.2 V.
A silicon oxide film has been used for a gate insulating film in a semiconductor transistor or a capacity insulating film in a semiconductor memory. A silicon oxide film with a thickness of 1 nm or less, however, has a current density of higher than 10 A/cm2 even for a direct tunneling current alone at an applied voltage of 1 V, and thus cannot be used in a next generation of semiconductor transistor or semiconductor memory. It has been, therefore, investigated to use a metal oxide with a higher dielectric constant as an insulating film.
For example, amorphous tantalum pentoxide (Ta2O5) is highly insulative with a relatively higher dielectric constant (25 to 26) and has been, therefore, investigated for its application to the above gate insulating film or a capacity insulating film. However, when applying a voltage of 1.2 V to an amorphous Ta2O5 sample with an equivalent oxide thickness tox of 1 nm (real film thickness of 6.5 nm), a leakage current density is 10−4 to 10−3 A/cm2. The above performance requirement for a gate insulating film or capacity insulating film is, therefore, not satisfied.
In other words, for using amorphous Ta2O5 in a gate insulating film or capacity insulating film, it is necessary to improve a dielectric constant while maintaining insulating performance; to improve insulating performance while maintaining a higher dielectric constant; or to improve both insulating performance and a dielectric constant.
For example, crystallization has been studied as an attempt to improving a dielectric constant of the above amorphous Ta2O5; specifically, β-Ta2O5 obtained after crystallization exhibited a dielectric constant ε of 35 which is 1.4 times of that for amorphous Ta2O5. However, crystallized Ta2O5 exhibits 105 times of leakage current density compared with that for amorphous Ta2O5 in the same electric field. The above requirement cannot be satisfied by simple crystallization.
For improving a dielectric constant of crystallized Ta2O5, for example, Applied Physics Letter, Vol. 74 (1999), p. 2370(J. Lin and co-workers) has described a technique that β-Ta2O5 with (001) orientation is deposited on a Ru substrate; specifically, β-Ta2O5 is deposited on Ru with (001) orientation, the substrate is heated initially at 350° C. for 3 minutes under N2O plasma and then at 800° C. for 1 minute under RTN (rapid thermal nitrization) to provide β-Ta2O5 with a dielectric constant of 100 or more.
However, because of leakage current deterioration due to the above crystallization, the film thickness of β-Ta2O5 must be 0.85 nm or more as an equivalent oxide thickness for achieving a leakage current density of 10−8 A/cm2 or less at an application voltage of 1V.
On the other hand, addition of tungsten oxide (WO3) or yttrium oxide (Y2O3) to amorphous Ta2O5 has been attempted for improving insulating performance of amorphous Ta2O5. Journal of Applied Physics, Vol. 75 (1994), p. 2538 (H. Fujikawa and co-workers) and Materials Research Society Symposium Proceedings, Vol. 378 (1995), p. 1025 (H. Fujikawa and co-workers) have reported that addition of 2 to 6 atom % of WO3 or 10 to 30 atom % of Y2O3 to amorphous Ta2O5 can significantly improve a leakage current in comparison with amorphous Ta2O5.
However, within a range where the amount of the dopant is sufficient to effectively reduce a leakage current, a dielectric constant is lower than that for amorphous Ta2O5. Thus, a real film thickness corresponding to an equivalent oxide thickness of 0.5 nm becomes approximately 2.4 nm while an electric field to a metal oxide at an applied voltage of 1.2 V becomes approximately 5 MV/cm. In such a status, a leakage current density for Ta2O5—WO3 or Ta2O5—Y2O3 is 10−5 A/cm2 or higher due to a tunneling current, so that a requirement for an insulating property is not met.
Thus, attempts to date for improving a dielectric constant of Ta2O5 and insulating performance have been failed. For other high dielectric materials, a requirement for an insulating property has not been met. For example, a real thickness of a BST film must be 20 nm or less for realizing 0.3 nm or less of an equivalent oxide thickness. Thinning of the BST film, however, causes deterioration in an insulating property. For example, the 46th Applied Physics Association Meeting proceeding No. 2, p. 883: 30p-ZS-13, Spring 1999 (Y. Fukuzumi and co-workers) has described that when a BST has a real thickness of 20 nm, a leakage current density is 10−6 A/cm2.
As described above, there has not been to date achieved a metal oxide meeting insulating performance and a dielectric constant required for a gate insulating film in a semiconductor transistor with a 0.05 μm generation of gate length and for a capacity insulating film in a semiconductor memory with a 0.13 μm generation of gate length.